Inductive heating method and apparatus

ABSTRACT

A method and apparatus ( 10 ) for the heating of convective material contained within the lumen of an inductive heating coil ( 12, 15, 16, 17 ) such that the cans ( 20 ) are rotated about their axes as they are heated. When the cans ( 20 ) are to be heated above the boiling point of their contents the cans ( 20 ) are passed through a rotary pressure lock ( 13, 19, 22 ) so that the heating of the contents of the cans ( 20 ) occurs under a super-atmospheric pressure.

TECHNICAL FIELD

The present invention relates to a method and apparatus for the heatingof canned products. More particularly the invention relates to such aprocess and device that utilises induction heating from an inductioncoil to heat the contents of the can by induction heating the metal can.

BACKGROUND ART

It is well know that many canned goods such as canned food. canned petfood, and the like require to be sterilised after canning to prevent thegrowth of toxic bacteria in the can. This is a relatively energeticprocess and considerable efforts have been made to improve the energyefficiency of processes for the heat sterilising of canned goods. Afurther requirement of such processes is that, at least in the case offoods for human consumption, it is important to retain the flavour,texture and nutrient status of the contents of the can. Typically thisrequires the minimum heating possible consistent with the propersterilisation of the food. A difficulty that then arises is that therate at which heat can be transmitted to the canned product will affectthe total heating time for the food. Many foods also have a tendency toburn onto the inside of the can if heated too quickly. The canner mustthen heat the can slowly to the sterilisation temperature which resultsin the contents of the can being maintained at all elevated temperaturefor considerably longer than is desirable for optimum taste, texture andnutrient status.

It has been proposed in U.S. Pat. No. 3,961,150 to heat canned goods bythe application of heat to the can by the use of induction coils. Thecans are described as being rotated in a reciprocating fashion beneath aflat induction coil. This arrangement was found in practice to beunsatisfactory. The time at which the contents of the can weremaintained at an elevated temperature was excessive due to the problemsof heat transfer rates, despite the reciprocating rotation of the cans.

French patent specification 2 501 631 describes an alternativearrangement in which cans are passed through the lumen of each of aplurality of helically wound induction coils. The cans are described asbeing rotated about their own axes between successive induction coils toallow homogenisation of the temperature of the contents of the cans. Itis believed by the present inventors that in this design, where the cansare not rotated while they are actually being heated that either therate of heating would have to be very low or there would be unacceptableburning of the contents against the can wall during the heating steps.

U.S. Pat. No. 4,256,922 describes the heating of loose foodstuffs on arotating metallic tube surrounded by a helically wound coil. In thiscase the beat transfer problems are rather different due to the factthat the food is free to tumble and rotate relative to the tubeunconstrained by a surrounding can.

The present invention is directed to an alternative method of heatingcanned product which offers an alternative to the known processesdescribed above.

DISCLOSURE OF INVENTION

The present invention relates to a method for the heating of aconvective material contained within a metallic can, the methodincluding the steps of passing the can longitudinally through the lumenof a helically wound induction coil, while simultaneously rotating oroscillating the can. The present further relates to apparatus for theheating of a convective material contained within a metallic can, thedevice including a helically wound induction coil adapted to induceheating in the wall of the can, means to pass cans longitudinallythrough the lumen of the coil, and means to cause the cans to rotateabout their own axes as they are passed through the coil.

The cans are preferably continuously rotated about a longitudinal axis.It is, however, within the ambit of the invention to oscillate or rotatethe cans about any axis or randomly by tumbling the can.

The present inventors have found that by passing the cans through thelumen of a coil it is possible, at least using preferred embodiments ofthe invention, to effectively heat the can at a high rate with increasedenergy efficiency. They have also found that if the cans aresimultaneously rotated at an appropriate rate it is possible with manycanned products to heat the product very rapidly while avoiding theproblems traditionally experienced with high heating rates.

It is preferred that the cans contain products that are liquid or aredisposed in a liquid. This allows the product to move reasonably readilyrelative to the surface of the can as it is rotated about its axis. Suchproducts are called convective products. Suitable convective productsinclude canned pasta in a sauce, canned vegetables in liquid, and somecanned pet foods.

The canned products are preferably rotated at least 50 rpm., morepreferably at least 80 rpm, and even more preferably 120 rpm. Inparticularly preferred embodiments of the invention the cans are rotatedat speeds of from 150 rpm to 200 rpm, preferably 180 rpm.

The calls for use in the process of the present invention must bemetallic or there will be no electromagnetic currents generated in thecall. Typically the cans will be formed of steel sheet coated with aprotective metal such as tin. Any container in the walls of whichheating may be induced by an oscillating electric current passingthrough the coil may be used in the present invention are for thepurposes of the present invention regarded as being metallic cans.

The induction coil is, or coils are, driven at a frequency of from 1 to500 kHz, preferably from 20 to 250 kHz. and more preferably from 50 to200 kHz. In particularly preferred embodiments of the invention thefrequency if from 60 to 180 kHz. The lower frequencies are cheaper togenerate but suffer from the disadvantage that they heat more slowly. Inany particular case there will be a trade off as between the cost of theprocess and the advantage to be obtained from more rapid processing ofthe canned product.

In a particularly preferred embodiment of the present invention the cansare passed through the lumen of an induction coil in a non metallictube. The calls may be pushed through the tube or conveyed through it inany other suitable way. The cans may be rotated at any desired speedsimply by rotating the tube at that speed. While this is a stronglypreferred mode of carrying out the present invention it is recognisedthat there are many other ways of carrying out the present invention.

If the canned product is a non-acid convective food it is essential thatit be heated at a temperature in excess of 100° C. if it is to beproperly sterilised. Heating to this temperature may cause the contentsto boil. The steam pressure from this boiling could cause the calls todistort or even rupture during heating. In order to avoid thepossibility of this distortion it is preferred that the heating of suchproducts be conducted in an atmosphere which is maintained at asuper-atmospheric pressure. The cans may be pre heated to, say, 90° C.at atmospheric pressure and then passed through a pressure lock into anatmosphere at an elevated pressure and the heating completed. Theheating step is most preferably carried out such that the contents ofthe can are heated at a rate of about 1° C. per second.

The heating is preferably carried out at the maximum rate possible untilthe can wall is at or above the maximum desired temperature. The can andits contents are then preferably allowed to rest for a period to allowequilibration of the contents of the can before further heating occurs.The can is most preferably rotated continuously throughout this restingperiod. If desired the rate of heating may be varied as the can ispassed through the lumen of the coil in order to provide a complete orpartial rest period. Alternatively there may be a plurality of coils inspaced apart array with a rest period between each coil.

Obviously in order to reduce the total time during which the contents ofthe can are elevated it is important to both raise and lower thetemperature of the contents of the can quickly. The present inventorshave found that if the cans that have been heated by the processaccording to the present invention are cooled while being rotated at therate previously described as being advantageous for the heating step itis possible to more rapidly cool the contents of the can. The coolingstep is preferably carried out under a stream of a cooling liquid suchas water. Initially this cooling will be carried out in asuper-atmospheric atmosphere. Once the temperature of the can and itscontents has been reduced to about 100° C. the can should be removed toan atmospheric pressure environment and the cooling continued.

BRIEF DESCRIPTION OF DRAWINGS

The following description of a preferred embodiment of the presentinvention is provided as an example of the invention and is describedwith reference to the accompanying drawings in which:

FIG. 1 is a schematic representation of a device for the carrying out ofthe process according to the present invention seen in plan view, and

FIG. 2 is a schematic vertical sectional view through a part of anapparatus according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The device 10 depicted in FIG. 1 is arranged to carry out a process ofheat sterilisation of canned food products. The food products arenon-acid convective foods. Typical examples include canned peas andspaghetti in tomato sauce. These foods are contained within conventionaltin plated steel cans.

The device 10 includes an inlet tube 11 arranged to be rotated about itslongitudinal axis at approximately 200 rpm. The tube 11 passes throughthe lumen of a first induction coil 12. This coil 12 is driven by asuitable source of an alternating current of 200 kHz. Cans 20 are forcedthrough the tube 11 by pusher means 24. These pusher means 24 areoperated to introduce cans 20 into tube 11 periodically andincrementally so that as one can 20 is introduced into the tube 11another is ejected from its other end, The passage time of cans 20through the tube 11 is so adjusted that the cans are heated to atemperature of about 90° C. by the time they reach the outlet end of thetube 11. The size of the coil 12 and the current put through it is soselected that the cans are heated at the maximum rate possible withoutthe contents of the can burning against the inside surface of the can.At a rotation speed of the tube 11 of 200 rpm it has been found that thecoil can be driven such that the cans 20 passing through the tube 11 areheated at a rate of 1° C./sec.

Cans 20 pushed out of the outlet end of tube 11 are pushed into a firsttransfer lock 13. This lock 13 transfers cans 20 from the atmosphericpressure environment of the tube 11 to a super-atmospheric environmentof a second tube 14. This super-atmospheric pressure is controlled bythe admission of compressed air to the tube 14 through air inlet 18. Thetube 14 is rotated about its longitudinal axis at the same speed as thetube 11. Cans 20 are pushed along tube 14 by further pusher means 25 sothat as each can 20 is introduced into the tube 14 another is beingejected from its discharge end. The tube 14 passes through the lumen ofthree successive induction coils 15, 16 and 17. These coils are drivenin the same manner as the coil 12. The can wall is heated to about 130°C. by the coil 15. The can then undergoes a rest period before beingfurther heated by coil 16. The process is then repeated and the can isheated for a final time by coil 17. The total heating regime is suchthat the contents of the can will have been sterilised in the minimumpossible time.

Cans 20 that have been sterilised in tube 14 are ejected into a secondtransfer lock 19. This lock transfers cans from the super-atmosphericenvironment of the heating tube 14 to the super-atmospheric environmentof a first cooling tube 21.

The tube 21 is rotated about its longitudinal axis just as the previoustubes 11 and 14 are. Similarly pusher means 26 are provided to transfercans through the tube 21. In the tube 21 the cans are sprayed (by meansnot shown) with water to cool them down, it has been found that thiscooling step is much more efficient at the very high rotation rate ofthe tube 21, and thus of the cans. The cooling means in tube 21 areadjusted so that the cans being ejected from the discharge end of thetube 21 have a temperature of about 100° C. They are discharged into athird transfer lock 22 which transfers the cans from thesuper-atmospheric atmosphere of the tube 21 to the atmosphericatmosphere of a cooling conveyor 23. This conveyor 23 is designed toallow the cans 20 to be rotated about their longitudinal axes and isalso provided with cooling means (not shown) such as water sprays toreduce the temperature of the cans 20 to a temperature a little aboveambient.

It has been found that by passing the cans through the lumen of thecoils 12, 15,16 and 17 it is possible to effectively sterilise thecontents of the cans while reducing the total time during which thecontents of the can are held at an elevated temperature considerably, ascompared with conventional sterilisatioin means. This in turn translatesinto the contents of the cans showing improved colour, organoleptic andnutrient status properties as compared with conventionally sterilisedcanned foods.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

What is claimed is:
 1. A method for the heating of a liquid or amaterial disposed in a liquid, contained within a metallic can, themethod including the steps of passing the can longitudinally through thelumen of a helically wound induction coil, heating the can within thelumen of the induction coil, while simultaneously rotating oroscillating the can.
 2. A method as claimed in claim 1 in which the canis continuously rotated about a longitudinal axis.
 3. A method asclaimed in claim 1 in which the can is rotated at least 50 rpm.
 4. Amethod as claimed in claim 3 in which the can is rotated at speeds offrom 150 to 200 rpm.
 5. A method as claimed in claim 1 in which the canis formed of steel sheet coated with a protective metal.
 6. A method asclaimed in claim 1 in which the can is passed through the lumen of aninduction coil in a non metallic tube.
 7. A method as claimed in claim 6in which the tube is pressurised.
 8. A method as claimed in claim 1 inwhich the induction coil is, or coils are, driven at a frequency of from1 to 500 kHz.
 9. A method as claimed in claim 8 in which the inductioncoil is, or coils are, driven at a frequency of from 60 to 180 kHz. 10.A method as claimed in claim 1 in which the cans is pre-heated to atemperature below the boiling point of the contents of the can atatmospheric pressure and then passed through a pressure lock into anatmosphere at an elevated pressure and the heating completed.
 11. Amethod as claimed in claim 1 in which the heating step is carried outsuch that the contents of the can are heated at a rate of about 1° C.per second.
 12. A method as claimed in claim 1 in which the can iscooled while being rotated.
 13. A method as claimed in claim 12 in whichthe cooling is carried out under a stream of a cooling liquid such aswater.
 14. A method as claimed in claim 12 in which the cooling iscarried out initially in a super-atmospheric atmosphere, once thetemperature of the can and its contents has been reduced to about 100°C. the can is removed to an atmospheric pressure environment and thecooling continued.
 15. Apparatus for the heating of a convectivematerial contained within a metallic can, the apparatus including ahelically wound induction coil adapted to induce heating in the wall ofthe can, means to cause cans to move longitudinally through the lumen ofthe coil, and means to cause the cans to rotate about their own axes asthey are passed through the coil.
 16. Apparatus as claimed in claim 15in which the means to cause the cans to rotate comprises a non-metallictube disposed within the linen of the coil and rotatable about its ownlongitudinal axis.
 17. Apparatus as claimed in claim 16 in which themeans to cause the can to move longitudinally through the tube comprisesa ram adapted to reciprocate between an extended position in which itapplies a pushing force to the end of a can to push it into the tube anda retracted position in which a can may be introduced between the ramand the tube.
 18. Apparatus as claimed in claim 15 in which the means tocause the cans to rotate is such that the cans are rotated at least 50rpm.
 19. Apparatus as claimed in claim 18 in which the means to causethe cans to rotate is such that the cans are rotated at speeds of from150 rpm to 200 rpm.
 20. Apparatus as claimed in claim 15 in which themeans are provided to pressurise the cans as they are being fed throughat least a part of the induction coil.
 21. Apparatus as claimed in claim15 in which the induction coil is, or coils are, driven at a frequencyof from 1 to 500 kHz.
 22. Apparatus as claimed in claim 21 in which theinduction coil is. or coils are, driven at a frequency of from 60 to 100kHz.
 23. Apparatus as claimed in claim 15 in which the apparatusincludes an induction coil to pre-heat the cans to a temperature belowthe boiling point of tile contents of the can at atmospheric pressure, apressure lock into an atmosphere at an elevated pressure. means to movethe can through the pressure lock and an induction coil to complete theheating of the can.
 24. Apparatus as claimed in claim 15 in which theinduction coil(s) is such that the contents of the can are heated at arate of about 1° C. per second.
 25. Apparatus as claimed in claim 15 inwhich cooling means are provided to cool the can while it is beingrotated.
 26. Apparatus as claimed in claim 25 in which the cooling meansincludes a stream of a cooling liquid such as water.
 27. A method asclaimed in claim 3 in which said can is rotated at least 80 rpm.
 28. Amethod as claimed in claim 27 in which said can is rotated at least 120rpm.
 29. A method as claimed in claim 4 in which the can is rotated at aspeed of substantially 180 rpm.
 30. A method as claimed in claim 8 inwhich the induction coil is, or coils are, driven at a frequency of from20 to 250 kHz.
 31. A method as claimed in claim 30 in which theinduction coil is, or coils are, driven at a frequency of from 50 to 200kHz.
 32. Apparatus as claimed in claim 18 in which the means to causethe cans to rotate is such that the cans are rotated at least 80 rpm.33. Apparatus as claimed in claim 32 in which the means to cause thecans to rotate is such that cans are rotated at least 120 rpm. 34.Apparatus as claimed in claim 19 in which the means to cause the cans torotate is such that cans are rotated at speeds of substantially 180 rpm.35. Apparatus as claimed in claim 21 in which the induction coil is, orcoils are, driven at a frequency of from 20 to 250 kHz.
 36. Apparatus asclaimed in claim 35 in which the induction coil is, or coils are, drivenat a frequency of from 50 to 200 kHz.